(1) Field of the Invention
The present invention relates to a power MOSFET structure and fabrication method thereof, and in particular, to a trench power MOSFET structure and fabrication method thereof.
(2) Description of the Prior Art
FIG. 1A to FIG. 1C schematically illustrates selected steps of the fabrication method for a traditional power MOSFET. An N-type power MOSFET is used as an example in the following. Referring to FIG. 1A, first, an N-type silicon substrate 110 is provided. Then, a mask is utilized to define the position of the gate trenches 120 wherein the gate trenches 120 are formed in the silicon substrate 110 through etching. Next, a gate dielectric layer 130 is formed on the exposed surfaces of the N-type silicon substrate 110. Thereafter, a polysilicon layer is deposited on the gate dielectric layer 130 and the gate trenches 120 are each filled with the polysilicon layer. Then, a portion of the polysilicon layer on the N-type silicon substrate 110 is etched back to form a polysilicon gate structure 140.
Next, referring to FIG. 1B, a passivation layer 131 is formed on the polysilicon gate structure 140. A blanket ion implantation method is utilized to implant the P-type impurities in the N-type silicon substrate 110 to form a heavy doped region (not shown in the figure). Then, referring to FIG. 1C, a thermal drive-in process is performed to have the implanted P-type impurities diffused downward forming a P-type body region 150 in the N-type silicon substrate 110. Afterward, the N-type impurities are implanted in the P-type body region 150 and another thermal drive-in process is applied to form a source doped region 160.
In order to improve device integration by shrinking the dimension of the MOSFET, the width of the gate trench 120 and source doped region 160 must be further reduced. However, as the width of the gate trench 120 is reduced, the gate impedance of the polysilicon gate 140 would be greatly increased. Relatively, the switch speed of the transistor would be negatively impacted as the switching loss would increase. As the width of the source doped region 160 shrinks, the conductive resistance of the source doped region 160 would increase causing the conduction loss to increase as well. Consequently, how to effectively improve the trench power MOSFET structure to have low gate impedance and low conductive resistance (Rds (ON)) becomes an urgent issue in the art to be resolved.
Therefore, fabricating a trench power MOSFET structure with high cell density and low conductive resistance is an important topic in the industry